Inhibitory mechanisms are known to be crucial in motor control, epilepsy, anaesthesia, and many other aspects of neural function. However, analyses of inhibitory mechanisms are complicated by uncertainties concerning the anatomy of inhibitory circuits and the location of inhibitory synapses on postsynaptic neurons. This project proposes to study the Renshaw inhibitory interneuron and its inhibitory connections with spinal motoneurons as a model to provide a better understanding of the mode of action of inhibitory circuits in the mammalian CNS. Identified Renshaw cells and target motoneurons will be intracellularly stained with horseradish peroxidase, using pipette microelectrodes, to directly determine the number and location of inhibitory synapses made by a Renshaw cell on a target motoneuron. This data will indicate whether Renshaw cells might act by inhibiting motoneuron firing in a global fashion (as a result of predominantly somatic locations of the synapses) or by more subtle inhibitory effects at specific dendritic locations, remote from the soma. The ultrastructure of identified Renshaw cell axons and terminals will be studied to determine their synaptic relationships, with identified motoneurons or other neural elements, and to provide insight into possible modes of junctional transmission at this synapse. Pharmacological analyses using blocking substances and analogues, applied both in vivo and in vitro, and immunocytochemical techniques, will be used to investigate the nature of the inhibitory neurotransmitter used by Renshaw cells.